Conventionally, head-mounted type display devices have different optimum optical specifications for different applications, such as for image viewing or mobile applications, and thus require separate devices to be prepared for the applications. For example, for image viewing applications, such specifications are preferable where a large-screen image can be displayed in front of a viewer within his/her visual field; for mobile applications, it is preferable to, for example, display an image at an end within the visual field while ensuring the outside visual field.
However, it is costly to prepare, and inconvenient to carry around different head-mounted type display devices depending on the number of usage required. Among others, and as will be more and more important in the future, in view of increasing availability of such devices for daily use and mobile applications, these devices can be used by the users in more diverse ways. There is a need for devices that can accommodate such diverse applications.
Therefore, in order to deal with different applications, one technology has been proposed to change viewing angles according to the application by providing a plurality of eyepiece optical systems in packages and selectively positioning the eyepiece optical systems in packages, as appropriate, in a head-mounted housing positioned in front of a user's eye (see, for example, Patent Document 1).
Another technology has also been proposed that allows switching between states by providing a liquid crystal shutter inside the optical path of the outside light: one in which only image light from an image display element is directed to an eyeball of the viewer and displayed as a non-see-through image, and the other in which outside light coming from ahead of the user is superposed on image light and the resulting image is displayed as a see-through image (see, for example, Patent Document 2).
Moreover, other technologies have been proposed that achieves see-through display using a light guide unit thinner than the pupil diameter (see, for example, Patent Document 3).